Integrated Amp Reviews

The first true high-end component I owned was an Audio Research SP-10. I reviewed this two-box, tubed preamplifier in the May 1984 issue of the English magazine Hi-Fi News & Record Review. "The SP-10 presented [recorded] information in a more coherent, less distorted manner than any preamp I've tried," I wrote in the review, concluding that "the SP-10 made me realize how many good records I owned." I purchased the SP-10 and brought the preamplifier with me when I moved to the US. Four decades later, I still have that SP-10. "Every now and again, when I want to be reminded of the magic it brings to my music, I set it up, plug in the tubes, and spend an evening spoiling my ears," I wrote for an article in Ken Kessler's 2020 book on the history of Audio Research.

The SP-10 was designed by a team led by William Zane Johnson, who founded Audio Research in Minnesota in 1970. After nearly four decades at the company's helm, he stepped aside in 2008, becoming chairman emeritus before passing away in 2011. But his legacy lives on, with a continuing succession of great-sounding tube preamplifiers and amplifiers. Several engineers contribute to the designs, but a consistent presence is Warren Gehl, Audio Research's long-term setup person and "Aural Evaluator," the "guardrail that keeps the company on the straight and narrow sonically," I was told by Dave Gordon, Audio Research's long-term keeper of the brand's institutional memory.

I was planning to review the latest product from Audio Research, the I/50 integrated amplifier, which costs $5500, earlier this year. However, with the uncertainty back then about the company's ownership, I postponed the review. When the news broke in June that Audio Research had been acquired by AR Tube Audio Corporation, a privately owned corporation that includes Valerio Cora of Canadian loudspeaker manufacturer Acora Acoustics as a director—see Industry Update in this issue—I unboxed the I/50 and set it up in my listening room.

The I/50
This is a tidy-looking tube amplifier. The black-gloss top panel has an aluminum surround, which is available in six anodized finishes. The review sample's was blue. Emerging from that top panel are two matched pairs of JJ "Eurotube" 6550WE output tubes and three 6922 small-signal tubes (footnote 1). A black cage at the panel's rear covers the AC power transformer and the two output transformers. At the front of that top panel are a ¼" headphone jack, a Power/Standby button, two rotary controls, and two black cylindrical pillars.

I wasn't sure what these pillars were until I plugged in the power cable and pressed the Power button. Inside the pillars, light-green alphanumeric "Lexi-tube" displays lit up and counted down from "50" in seconds. Once the numbers reached zero, the display briefly showed which input was last selected—"B" and "L" for Balanced, for example—and then showed the prior volume setting, on a scale from "00" to "45." Pressing the volume control at the right front of the top panel unmutes the amplifier, the Lexitubes brighten, and a new input can be selected by rotating the knob on the left front of the panel. Pressing the Power button a second time puts the amplifier in Standby mode, and the displays show a single dot. (In this mode, the tubes are powered down, but the control circuitry remains active.) The manual warns that after the amplifier has been put in standby, the user should wait at least three minutes before turning it back on.

There are four inputs on the rear panel, one balanced pair on XLR jacks and three single-ended pairs on RCA jacks. The first RCA pair, labeled "SE1," serves as input for the optional MM/high-output MC phono module ($750) if it is installed. (There is also an optional DAC module for $1000.) Otherwise, SE1 is line level. The second input, SE2, is a standard line-level input. The third, labeled "SE3/PassThru," can be used either with the volume control active (as a standard line-level input) or set to unity gain to allow the connection of a secondary preamplifier such as a multichannel processor. Pressing and rotating the source selector allows the Phono and PassThru settings to be changed when these inputs are active.

There are three heavy-duty speaker binding posts for each channel, marked "0Ω," "4Ω," and "8Ω," which connect to the transformer taps intended for nominally 4 ohm and 8 ohm loudspeakers. The posts for each channel are vertically offset from one another, but with the adjacent IEC power jack, space is tight. Care needs to be taken when connecting speaker cables unless they are fitted with banana plugs.

All the top-panel functions, including setting the display brightness, are duplicated on a slim metal remote control. I didn't see any way to change its battery, if that becomes necessary, but Dave Gordon enlightened me: "Just push a pin into the hole on the back of the remote and it will push out the control board."

The Audio Research I/50 is a combination of classic tube amplifier design with 21st century microprocessor-controlled functionality.

Setup
The review sample wasn't fitted with the optional Phono or DAC modules (footnote 2). After I installed the tubes, following the numbering in the packing list and making sure that the supplied tube-damping ring was fitted to the central 6922, the input tube, I used two different speakers with the Audio Research amplifier, both standmounts: my long-term reference KEF LS50s and a pair of GoldenEar BRXes.

To operate the I/50, I exclusively used its ergonomically satisfying remote control. Sources were either an MBL N31 CD player/DAC, connected to the I/50's balanced input with Ayre/Cardas Signature cables, or the HiFi Rose 250A streaming DAC connected to SE2 with unbalanced Canare cables.

I set both D/A processors' volume controls to their maximum and sent them audio data with Roon via an Ethernet connection from my router. (As always, I used Roon's parametric equalizer to flatten the speakers' midbass region and extend the bass.) I also briefly used the Mytek Brooklyn Bridge II that Tom Fine reviews elsewhere in this issue.

Listening
As always, I started my auditioning with some of my own recordings, using the amplifier's 8 ohm output taps. Although neither loudspeaker is a particularly demanding load, with impedances that remain above 6 ohms over much of the audioband, the balance was somewhat forward-sounding with a touch of glare in the midrange—the latter especially with the GoldenEars. The amplifier's manual cautions that "it is important to use as close as possible an impedance match between the amplifier and speaker for optimum transfer of power to the speaker with minimum distortion." I changed to the 4 ohm output taps. After adjusting the volume to compensate for the lower gain—see the Measurements sidebar—I heard considerable improvement, and I used that output for the remainder of my auditioning.

---

Footnote 1: Audio Research says that the 6922 tubes should be replaced before 4000 hours of use; the 6550WE tubes should be replaced after approximately 2000 hours. When power tubes or the tube set are replaced, the I/50 must be biased by a trained technician to ensure proper operating values.

Footnote 2: I was sent the DAC module toward the end of the review period. I will write about the module's performance in a future issue.